1. Field of the Invention
The present invention relates to a power amplifying circuit (e.g., a class D power amplifier).
2. Description of Related Art
(First Related Art)
A short circuit in the output of a push-pull circuit or the like including two transistors connected in series is usually detected by using a sensing resistor. For instance, JP-A-11-239029 (hereinafter referred to as Patent Document 1) discloses and proposes a technique as shown in FIG. 1 of the document, in which a sensing resistor connected to a source of an N-channel field effect transistor is used for sensing current flowing in a complementary circuit, and a switch circuit is driven to cut off a power supply if the sensed current is not smaller than a predetermined threshold value.
However, the above-mentioned conventional short circuit detection circuit needs an expensive sensing resistor, so it causes a cost increase. In addition, since current flows in the sensing resistor of the above-mentioned conventional short circuit detection circuit, undesired power loss is caused.
(Second Related Art)
Conventionally, a class D power amplifier to which an analog signal (e.g., analog audio signal) is input has a limiter circuit for limiting its output power.
Conventional techniques of the above-mentioned limiter circuit are disclosed in JP-A-2006-5741 and JP-A-10-335961 (hereinafter referred to as Patent Documents 2 and 3, respectively).
The Patent Document 2 discloses and proposes a limiter circuit for clipping an input signal applied to an input terminal at a constant voltage by a control signal that is applied externally. The limiter circuit includes a control element connected between a power supply terminal and the input terminal and a control circuit that turns off the control element normally and turns on the control element when the input signal exceeds the constant voltage.
In addition, the Patent Document 3 discloses and proposes a bass amplifying system for driving a sub woofer, which includes a preamplifier for amplifying an input signal, a waveform shaping circuit for shaping a waveform of an output signal of the preamplifier, a power amplifier for amplifying an output of the waveform shaping circuit, and a power supply voltage supplying circuit for supplying the preamplifier with a power supply voltage of an absolute value smaller than a power supply voltage that is supplied to the power amplifier.
According to the above-mentioned conventional techniques, an increase in distortion caused by output saturation or damage to a speaker can surely be prevented even if an excessive input signal is applied.
However, the clipped waveform in the conventional technique described in the Patent Document 2 is not a soft-clipped waveform, so the output signal contains a lot of harmonic components as a result. Therefore, the speaker may produce an unusual grating sound.
In addition, since a limit level of the output power in the conventional technique described in the Patent Document 2 responds to variable control of a control voltage relatively too sensitively, it is not always easy to set a desired output power accurately.
In addition, since upper and lower limit levels are set by using a control signal in the conventional technique described in the Patent Document 2, it is necessary to adjust circuit constants in order to realize a symmetrical clipping with respect to a bias voltage of an operational amplifier.
On the other hand, the conventional technique described in the Patent Document 3, which realizes the soft clip by controlling the power supply voltage of the preamplifier, is different from the present invention concerning the essential structure.
(Third Related Art)
Conventionally, various techniques are proposed for preventing damage to a speaker or burning thereof by detecting a DC output and protecting the same in a power amplifying circuit for amplifying an audio signal. Some examples thereof are disclosed in JP-A-2003-258575, JP-A-2000-152367 and JP-A-2000-151297 (hereinafter referred to as Patent Documents 4, 5 and 6, respectively), as well as in “YAMAHA Digital Amplifier IC”, [online], Apr. 10, 2007, YAMAHA CORPORATION, [Searched on Nov. 1, 2007], the Internet <URL: http://www.yamaha.co.jp/news/2007/07041001.html> (hereinafter referred to as Non-patent Document 1).
The Patent Document 4 proposes a BTL amplifying system that detects and decides whether or not an offset is generated so as to prevent a load such as a speaker from damage. However, there is a problem that the BTL amplifying system proposed in the Patent Document 4 cannot be applied to a class D amplifier since it detects and protects the DC output by detecting and deciding whether or not an offset is generated.
The Patent Document 5 proposes a BTL power amplifier that detects a clip of an output voltage waveform so as to protect a speaker. However, there is a problem that the BTL power amplifier proposed in the Patent Document 5 cannot be applied to a class D amplifier since it detects and protects the DC output by detecting a clip of an output voltage waveform.
The Patent Document 6 proposes a fault detection circuit that detects an output current difference between output transistors of two BTL amplifying circuits for driving different speakers respectively so as to protect the speakers. However, there is a problem that the fault detection circuit proposed in the Patent Document 6 needs two BTL amplifying circuits for driving individual speakers, so it is difficult to protect the speaker by a single BTL amplifying circuit driving one speaker.
The Non-patent Document 1 discloses a block diagram as shown in FIG. 78, and a class D amplifier having a DC output detecting function is disclosed. However, a concrete circuit structure for detecting a DC output is not disclosed or suggested at all, so it is not clear how the DC output detecting function is realized with which circuit structure.
Furthermore, although a DC output of a class D amplifier is usually generated by a ground fault or a short-to-power fault of the input terminal or the like of the class D amplifier in a BTL output type, it may be generated also by an abnormal state that occurred in the driver. Therefore, it is desirable that a DC output generated by an abnormal state that occurred in the driver can be detected. In addition, it is also desirable that detection time for the DC output detecting function can be changed in accordance with the performance of the speaker that is connected. However, the conventional techniques described above cannot satisfy these requirements.